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Biomechanics

Biomechanics is the science concerned with the
internal and external forces acting on the human body and the effects produced
by these forces.

Kinetics is a study of the cause of motion, namely forces and torques e.g. forces between the feet and the ground when jumping and Kinematics is the study of movement with reference to the amount of time taken
to carry out the activity.

Distance and displacement

Distance (length of the path a body
follows) and displacement (length of a straight line joining the start and
finish points) are quantities used to describe
a body's motion. e.g. in a 400m race on a 400m track the distance is 400 metres but their displacement will be zero metres (start and finish at the same point).

Speed and velocity

Speed and velocity describe the rate at which a body moves
from one location to another. Average speed of a body is obtained by dividing the distance by
the time taken and average velocity is obtained by dividing the
displacement by the time taken e.g. a swimmer in a 50m race in a 25m
length pool who completes the race in 71 seconds - distance is 50m and
displacement is 0m (swimmer is back where they started) so speed is 50/71=
0.70m/s and velocity is 0/71=0 m/s

Speed and Velocity = distance travelled ÷ time
taken

Acceleration

Acceleration is defined as the rate at which velocity
changes with respect to time.

Uniformly accelerated motion

When a body experiences the same acceleration throughout an
interval of time, its acceleration is said to be constant or uniform and the following equations apply:

Final velocity = initial velocity + (acceleration x time)

Distance = (initial velocity x time) + (½ x acceleration
x time²)

Moment of force (torque)

The moment of force or torque (τ) is defined as the application of a
force at a perpendicular distance to a joint or point of rotation.

Torque (τ = rFsin θ ) depends on three quantities:

the length of the lever arm connecting the axis to the point of force application (r)

the force applied (F)

the angle between the force vector and the lever arm (sin θ)

Angular Kinematics

Angular distance and displacement

When a rotating body moves from one position to another, the
angular distance through which it moves is equal to the length of the angular
path. The angular displacement that a rotating body experiences is equal to the angle between the initial and final position of the body.

Angular movement is usually expressed in radians where 1 radian =
57.3°

Angular speed, velocity and acceleration

Angular Momentum

Angular momentum is defined as: angular velocity x moment of
inertia

The angular momentum of a system remains constant throughout a
movement provided nothing outside of the system acts with a turning moment on it.
This is known as the Law Conservation of Angular Momentum. (e.g. if a skater, when already spinning, moves their arms out to the side, then the rate of spin will
change but the angular momentum will stay the same).

Linear Kinetics

Kinetics is concerned with what causes a body to move.

Momentum, inertia, mass, weight and force

Momentum: mass x velocity

Inertia: the
reluctance of a body to change whatever it is doing

Mass: the quantity of matter of which a body is
composed of - not affected by gravity - measured in kilograms (kg)

Weight: force due to gravity -9.81m/s²

Force: a pushing or pulling action that causes a
change of state (rest/motion) of a body is proportional to mass x
acceleration. It is measured in Newtons (N) where 1N is the force that will
produce an acceleration of 1 m/s² in a body of 1kg mass

The classification of external or internal forces depends
on the definition of the 'system'. In biomechanics, the body is seen as the
'system' so any force exerted by one part of the system on another part of the 'system' is known as
an internal force all other forces are external.

First Law: Every body continues in its state of rest
or motion in a straight line unless compelled to change that state by external
forces exerted upon it.

Second Law: The rate of change of momentum of a body
is proportional to the force causing it and the change takes place in the
direction in which the force acts

Third Law: To every action there is an equal and
opposite reaction OR for every force that is exerted by one body on
another there is an equal and opposite force exerted by the second body on the
first

Any two particles of matter attract one another with a
force directly proportional to the product of their masses and inversely
proportional to the square of the distance between them

Kinetic Energy and Power

Kinetic energy is the mechanical energy possessed by a moving
object.

Kinetic Energy = ½ x mass x velocity² (joules)

Power is defined as the rate at which energy is used or created
from other forms

Power = energy used ÷ time taken

Power = (force x distance) ÷ time taken

Power = force x velocity

Angular Kinetics

Translation and couple

A force that acts through the centre of a body result in
movement (translation). A force whose line of action which does not pass through the body's
centre of gravity is called an eccentric force and results in movement and
rotation.

Example - if you push through the centre of an object it will move
forward in the direction of the force. if you push to one side of
the object (eccentric force) it will move forward and rotate.

A couple is an arrangement of two equal and opposite forces that
cause a body to rotate.

Levers

A lever is a rigid structure, hinged at one point and to which
forces are applied at two other points. The hinge is known as
the fulcrum. The two forces forces that act on the lever are the weight
that opposes movement and a force that causes movement. For more
details see the page on Levers.

Bernoulli Effect

If an object has a curved top and flat bottom
(e.g. the wing of an aircraft), the air will have further to travel over the top of the wing than
the bottom. For the two airflows to reach the rear of the wing at the same
time the air flowing over the top of the wing will have to flow faster resulting in less pressure above the wing (air is thinner) than
below it and the aircraft will lift. This is known as the Bernoulli
effect.